linux-kernel - Re: [RFC][PATCH 12/13] stop

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Message-ID: <20150625191802.GX19282@twins.programming.kicks-ass.net>
Date:	Thu, 25 Jun 2015 21:18:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, der.herr@...r.at, dave@...olabs.net,
	riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

On Tue, Jun 23, 2015 at 07:24:16PM +0200, Oleg Nesterov wrote:
> IOW. Suppose we add ->work_mutex into struct cpu_stopper. Btw,
> I think we should move all per-cpu variables there...
> 
> Now,
> 
> 	lock_stop_cpus_works(cpumask)
> 	{
> 		for_each_cpu(cpu, cpumask)
> 			mutex_lock(per_cpu(cpu_stopper_task, cpu).work_mutex);
> 	}
> 
> 	unlock_stop_cpus_works(cpumask)
> 	{
> 		for_each_cpu(cpu, cpumask)
> 			mutex_lock(...);
> 	}
> 
> which should be used instead of stop_cpus_mutex. After this change
> stop_two_cpus() can just use stop_cpus().

Right, lockdep annotating that will be 'interesting' though. And
stop_two_cpus() then has the problem of allocating a cpumask. Simpler to
let it keep 'abuse' the queueing spinlock in there.

> Off-topic. Can't we make __stop_machine() static? The only caller,
> _cpu_down() can safely call stop_machine(), get_online_cpus() is
> fine under cpu_hotplug_begin().

Can do I think.
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